Custom data field for bots

As a bot developer, I think in many cases it would be useful to have a small data field sent with the message that only the message sender can see. Or actually the notes field to be accessible for bots too. That way without a db it would be possible to store a small chunk of data such as economy (the user's money), some perks, bans etc. And the message data field would also help holding "paginator" states.